Manufacturing process for coated polyester film

ABSTRACT

A method of preventing or minimising the formation of haze in a biaxially oriented polyester film during the annealing of said film above its glass transition temperature, said method comprising: (a) selecting a biaxially oriented polyester film having glass transition temperature (Tg (° C.)); (b) disposing a coating on one or both surfaces of said biaxially oriented film; (c) annealing said coated biaxially oriented polyester film at a temperature above its glass transition temperature, wherein said coating composition is selected from: (i) an organic coating comprising a low molecular weight reactive diluent; an unsaturated oligomer; a solvent; and a photoinitiator; (ii) an organic/inorganic hybrid coating comprising a low molecular weight reactive component and/or an unsaturated oligomeric component; a solvent; and inorganic particles, and optionally further comprising a photoinitiator; (iii) a predominantly inorganic hardcoat comprising inorganic particles contained in a polymerisable predominantly inorganic matrix; and (iv) a composition comprising a cross-linkable organic polymer selected from polyethylene imine (PEI), polyester and polyinylalcohol (PVOH), and a cross-linking agent.

The present invention relates to improvements in polyester film to makeit more suitable in applications such as electronic, photonic andoptical assemblies or structures. During the manufacture of suchassemblies or structures, the polyester film substrate is thermallyprocessed at elevated temperatures, and this thermal treatment canresult in the production of cyclic oligomers within the film, which canmigrate to, and contaminate, the film surface, causing the film tobecome hazy.

The migration or diffusion of small molecules through a solid polymericmedium has been intensively studied and characterised, because it formsthe underlying principle of a number of important commercialapplications. For example, containment of liquids and gases either instatic reservoirs and tanks, or during transport along tubing and pipesclearly relies on very slow or zero diffusion by the mobile materialthrough the walls of the plastic housing. Similarly for the purposes ofpackaging and shelf-life of materials, from food, drink andpharmaceuticals to commercial goods such as chemicals and electronics,plastic materials are required for packaging in which diffusion ofspecific gases or liquids is extremely slow. In the case of separationor purification using membranes the differential diffusion or transportof several gases or liquids through a solid membrane material is againthe basis of permeation technology.

The phenomenon of diffusion of small molecules through a polymer mediumalso operates when issues of contamination arise. For example themigration of residual monomer out of a formed polymeric article canoften cause unwanted contamination (A. R. Berens, C. A. Daniels, Polym.Eng. Sci., 16, 552 (1976)). Thus, industrial process steps have beendeveloped which extract unreacted monomer from plastic before its use infinal fabrication (V. T. Stannet, Polym. Eng. Sci., 18, 1129 (1978)).Similarly, if the polymer manufacturing process involves polymerisationor treatment in solution, traces of residual solvent, which remainsoccluded in the plastic will leach out of the material over time (T. J.Stanley, M. M. Alger, Ind. Eng. Chem. Res., 28, 865 (1989). The third,common low molecular weight material which is often found to migrate outof a commercial polymer, or article fabricated from it, is aplasticiser. In contrast to the former examples, a plasticiser isintentionally added or compounded into the plastic in order to modifyits mechanical properties. However, it too will diffuse out of thepolymer with time and present similar issues around contamination. Forsuch systems, of which plasticised PVC is a familiar example, routes toimprovement involve the development of new plasticisers (which are lessmobile in the polymeric environment or more benign as a contaminant) orthe use of physical barriers, impermeable coatings which prevent loss ofthe plasticisers from the plastic to its environment (A. Jayakrishnan etal., J. Appl. Polym. Sci., 56, 1187 (1995)).

Polyester materials represent an unusual example of a system which cancontain low molecular weight molecules. These species are believed tooriginate from and exist in equilibrium with the parent polymer attemperatures above its melting range (Tm). As a consequence theyoriginate at the manufacturing stage of the polymer. The low molecularweight material is predominantly a cyclic oligomer (trimer) and has achemical structure identical to that of the high polymer. Its regularstructure allows the cyclic oligomer to crystallise easily, thus undercertain thermal treatments the material will diffuse to the surface of asolid polyester and crystallise (S. Reichlmaier et al., J. Vac. Sci.Tech., A13, 1217 (1995); and Y. Kawahara et al., Macromol. mater. Eng.,291, 11 (2006)). When this behaviour interferes with the performance ofthe polyester or its environment, the presence of the cyclic oligomersis seen as undesirable contamination, as a problem which requires asolution. Two approaches have been described to prevent the migration ofa polyester cyclic oligomer to the surface of a polyester article eitherduring further processing or in final service. In the first case,articles such as film or fibre are produced from a polyester rawmaterial with a reduced inital content of cyclic oligomer. (Kawaharaibid; U.S. Pat. No. 6,020,056; and U.S. Pat. No. 6,054,224) This issuccessful when the fibre or film manufacturing process subjects thepolyester to melt temperatures for a duration which is too short toallow the equilibrium level of oligomer to re-establish in the system.The second case involves laminating or coextruding an outer layer ofplastic to the article which operates as a barrier to diffusion by thecyclic species to the new surface. This is successful if the laminatedsurface has properties which match or surpass those of the originalpolyester but often carries the disadvantage of higher cost (U.S. Pat.No. 5,545,364). To date, however, the use of a coating technology hasnot been previously applied to perform this function on polyesterarticles.

The polyester article of particular interest in the present applicationis film used in the field of flexible electronic or opto-electronictechnology, as disclosed in, for instance, WO-A-03/022575. Inparticular, the film is a substrate on which electronic circuitry ismanufactured and mounted in order to drive the electronic operation ofthe flexible device. The component which comprises the flexiblesubstrate and circuitry is often described as a backplane. During thecourse of fabrication of the final backplane, the substrate is oftenexposed to conditions of elevated temperature for extended times. Thedevelopment of physical contamination on its surface through the processof diffusion and crystallisation by cyclic oligomers is undesirable andcan be immediately and conveniently recognised and measured as theappearance of haze on the film. There is a need to provide an improvedbarrier to the diffusion of cyclic oligomer from polyester filmsubstrates which are exposed to conditions of elevated temperature forextended times in this technology.

The object of the present invention is to minimise or prevent theformation of haze in a polyester film substrate during the thermalprocessing thereof at elevated temperatures, for instance during themanufacture of electronic, photonic and optical assemblies orstructures. It is a further object of this invention to minimise orprevent the formation of haze in a polyester film which has excellentdimensional stability.

The present invention provides the use of a coating composition selectedfrom:

(i) an organic coating comprising a low molecular weight reactivediluent; an unsaturated oligomer; a solvent; and a photoinitiator;(ii) an organic/inorganic hybrid coating comprising a low molecularweight reactive component and/or an unsaturated oligomeric component; asolvent; and inorganic particles, and optionally further comprising aphotoinitiator;(iii) a predominantly inorganic hardcoat comprising inorganic particlescontained in a polymerisable predominantly inorganic matrix; and(iv) a composition comprising a cross-linkable organic polymer selectedfrom polyethylene imine (PEI), polyester and polyinylalcohol (PVOH), anda cross-linking agent,for the purpose of preventing or minimising the formation of haze in abiaxially oriented polyester film during the annealing of said filmabove the glass transition temperature (Tg (° C.)) of said film.The present invention further provides a method of preventing orminimising the formation of haze in a biaxially oriented polyester filmduring the annealing of said film above its glass transitiontemperature, said method comprising:(a) selecting a biaxially oriented film having glass transitiontemperature (Tg (° C.));(b) disposing a coating on one or both surfaces of said biaxiallyoriented film;(c) annealing said coated biaxially oriented polyester film at atemperature above its glass transition temperature, wherein said coatingcomposition is selected from:(i) an organic coating comprising a low molecular weight reactivediluent; an unsaturated oligomer; a solvent; and a photoinitiator;(ii) an organic/inorganic hybrid coating comprising a low molecularweight reactive component and/or an unsaturated oligomeric component; asolvent; and inorganic particles, and optionally further comprising aphotoinitiator;(iii) a predominantly inorganic hardcoat comprising inorganic particlescontained in a polymerisable predominantly inorganic matrix; and(iv) a composition comprising a cross-linkable organic polymer selectedfrom polyethylene imine (PEI), polyester and polyinylalcohol (PVOH), anda cross-linking agent.

As used herein, the term “annealed” or “annealing” refers to the step ofheating the film at elevated temperatures above its Tg, and relates toconditions experienced by the film in subsequent post-processing orfabrication, for instance in the manufacture of the backplanes referredto hereinabove. In one embodiment, the annealing is conducted at atemperature T_(a) (° C.) above Tg where Tg<T_(a)≦Tg+100 (° C.). In afurther embodiment, the annealing is conducted for a time t afterthermal equilibrium where 1 hour≦t≦72 hours, typically wherein 1hour≦t≦48 hours, and more typically 1 hour≦t≦24 hours. Following saidannealing, the film is then cooled.

The inventors have unexpectedly found that the presence of the coatingcompositions disclosed herein, and particularly the hard-coatingcompositions, reduce the level of haze induced by high-temperatureprocessing.

In one embodiment, the coating is present on both sides of the polyestersubstrate.

The term polyester as used herein includes a polyester homopolymer inits simplest form or modified, chemically and/or physically. Inparticular, the material to be treated by the annealing process is abiaxially oriented polymeric film comprising a layer of polyester orcopolyester derived from:

-   (i) one or more diol(s);-   (ii) one or more aromatic dicarboxylic acid(s); and-   (iii) optionally, one or more aliphatic dicarboxylic acid(s) of the    general formula C_(n)H_(2n)(COOH)₂ wherein n is 2 to 8,    wherein the aromatic dicarboxylic acid is present in the    (co)polyester in an amount of from about 80 to about 100 mole %    based on the total amount of dicarboxylic acid components in the    (co)polyester. A copolyester may be a random, alternating or block    copolyester.

The thickness of the film is preferably from about 12 to about 250 μm,more preferably from about 12 to about 150 μm, and typically is about25-125 μm in thickness. The film is self-supporting by which is meantcapable of independent existence in the absence of a supporting base.

The polyester is obtainable by condensing said dicarboxylic acids ortheir lower alkyl (up to 6 carbon atoms) diesters with one or morediols. The aromatic dicarboxylic acid is preferably selected fromterephthalic acid, isophathalic acid, phthalic acid, 2,5-, 2,6- or2,7-naphthalenedicarboxylic acid, and is preferably terephthalic acid or2,6-naphthalenedicarboxylic acid, preferably 2,6-naphthalenedicarboxylicacid. The diol is preferably selected from aliphatic and cycloaliphaticglycols, e.g. ethylene glycol, 1,3-propanediol, 1,4-butanediol,neopentyl glycol and 1,4-cyclohexanedimethanol, preferably fromaliphatic glycols. Preferably the copolyester contains only one glycol,preferably ethylene glycol. The aliphatic dicarboxylic acid may besuccinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid,azeleic acid or sebacic acid. Preferred homopolyesters are polyesters of2,6-naphthalenedicarboxylic acid or terephthalic acid with ethyleneglycol. A particularly preferred homopolyester is poly(ethylenenaphthalate), and particularly polyesters of 2,6-naphthalenedicarboxylicacid with ethylene glycol.

Formation of the polyester is conveniently effected in a known manner bycondensation or ester interchange, generally at temperatures up to about295° C. For instance, the preferred PEN polyester can be synthesised bycondensing 2,5-, 2,6- or 2,7-naphthalenedicarboxylic acid, preferably2,6-naphthalenedicarboxylic acid, or a lower alkyl (up to 6 carbonatoms) diester thereof, with ethylene glycol. Typically,polycondensation includes a solid phase polymerisation stage. The solidphase polymerisation may be carried out on a fluidised bed, e.g.fluidised with nitrogen, or on a vacuum fluidised bed, using a rotaryvacuum drier. Suitable solid phase polymerisation techniques aredisclosed in, for example, EP-A-0419400 the disclosure of which isincorporated herein by reference. In one embodiment, the PEN is preparedusing germanium catalysts which provide a polymeric material having areduced level of contaminants such as catalyst residues, undesirableinorganic deposits and other byproducts of the polymer manufacture. The“cleaner” polymeric composition promotes improved optical clarity andsurface smoothness. Preferably, PEN has a PET-equivalent intrinsicviscosity (IV) of 0.5-1.5, preferably 0.7-1.5, and in particular0.79-1.0. An IV of less than 0.5 results in a polymeric film lackingdesired properties such as mechanical properties whereas an IV ofgreater than 1.5 is difficult to achieve and would likely lead toprocessing difficulties of the raw material.

The Tg of a preferred homopolyester, PEN, is generally acknowledged tobe 120° C., while that of the other preferred homopolyester, PET, isgenerally acknowledged to be 80° C. Copolyesters can exhibit Tg valueseither below or above those of the parent homopolymer depending on thenature of the comonomer which is incorporated. A film made from thepolyester may exhibit Tg values higher than that of the polyester rawmaterial, depending on the crystallinity of the film. Thus, as thecrystallinity of the film increases, the polyester chains in theamorphous regions of the film become more restricted in their movement,meaning that the glass transition is observed at higher temperatures.For the avoidance of doubt, the annealing temperature (T_(a)) of theprocess of the present invention is dependent on the Tg of the polyesterfilm, rather than the polyester raw material.

Formation of the film may be effected by conventional techniqueswell-known in the art. Conveniently, formation of the film is effectedby extrusion, in accordance with the procedure described below. Ingeneral terms the process comprises the steps of extruding a layer ofmolten polymer, quenching the extrudate and orienting the quenchedextrudate in at least one direction.

The film is biaxially-oriented. Orientation may be effected by anyprocess known in the art for producing an oriented film, for example atubular or flat film process. Biaxial orientation is effected by drawingin two mutually perpendicular directions in the plane of the film toachieve a satisfactory combination of mechanical and physicalproperties.

In a tubular process, simultaneous biaxial orientation may be effectedby extruding a thermoplastics polyester tube which is subsequentlyquenched, reheated and then expanded by internal gas pressure to inducetransverse orientation, and withdrawn at a rate which will inducelongitudinal orientation.

In the preferred flat film process, the film-forming polyester isextruded through a slot die and rapidly quenched upon a chilled castingdrum to ensure that the polyester is quenched to the amorphous state.Orientation is then effected by stretching the quenched extrudate in atleast one direction at a temperature above the glass transitiontemperature of the polyester. Sequential orientation may be effected bystretching a flat, quenched extrudate firstly in one direction, usuallythe longitudinal direction, i.e. the forward direction through the filmstretching machine, and then in the transverse direction. Forwardstretching of the extrudate is conveniently effected over a set ofrotating rolls or between two pairs of nip rolls, transverse stretchingthen being effected in a stenter apparatus. Stretching is generallyeffected so that the dimension of the oriented film is from 2 to 5, morepreferably 2.5 to 4.5 times its original dimension in the or eachdirection of stretching. Typically, stretching is effected attemperatures higher than the Tg of the polyester, preferably about 15°C. higher than the Tg. Greater draw ratios (for example, up to about 8times) may be used if orientation in only one direction is required. Itis not necessary to stretch equally in the machine and transversedirections although this is preferred if balanced properties aredesired.

A stretched film may be, and preferably is, dimensionally stabilised byheat-setting under dimensional support at a temperature above the glasstransition temperature of the polyester but below the meltingtemperature thereof, to induce crystallisation of the polyester. Duringthe heat-setting, a small amount of dimensional relaxation may beperformed in the transverse direction, TD by a procedure known as“toe-in”. Toe-in can involve dimensional shrinkage of the order 2 to 4%but an analogous dimensional relaxation in the process or machinedirection, MD is difficult to achieve since low line tensions arerequired and film control and winding becomes problematic. The actualheat-set temperature and time will vary depending on the composition ofthe film and its desired final thermal shrinkage but should not beselected so as to substantially degrade the toughness properties of thefilm such as tear resistance. Within these constraints, a heat settemperature of about 180° to 245° C. is generally desirable.

The film may also, and indeed preferably is, further stabilized throughuse of an online relaxation stage. Alternatively the relaxationtreatment can be performed off-line. In this additional step, the filmis heated at a temperature lower than that of the heat-setting stage,and with a much reduced MD and TD tension. Film thus processed willexhibit a smaller thermal shrinkage than that produced in the absence ofsuch post heat-setting relaxation.

In one embodiment, heat-setting and heat-stabilisation of the biaxiallystretched film is conducted as follows. After the stretching steps havebeen completed, heat-setting is effected by dimensionally restrainingthe film at a tension in the range of about 19 to about 75 kg/m,preferably about 45 to about 50 kg/m of film width, using a heat-settemperature preferably from about 135° to about 250° C., more preferably235-240° C. and a heating duration typically in the range of 5 to 40secs, preferably 8 to 30 secs. The heat-set film is then heat-stabilisedby heating it under low tension, preferably such that the tensionexperienced by the film is less than 5 kg/m, preferably less than 3.5kg/m, more preferably in the range of from 1 to about 2.5 kg/m, andtypically in the range of 1.5 to 2 kg/m of film width, typically using atemperature lower than that used for the heat-setting step and selectedto be in the range from about 135° C. to 250° C., preferably 190 to 250°C., more preferably 200 to 230° C., and more preferably at least 215°C., typically 215 to 230° C., and for a duration of heating typically inthe range of 10 to 40 sec, with a duration of 20 to 30 secs beingpreferred.

The heat-set, heat-stabilised film exhibits a very low residualshrinkage and consequently high dimensional stability. Preferably, thefilm exhibits a coefficient of linear thermal expansion (CLTE) withinthe temperature range from 40° C. to +100° C. of less than 40×10⁻⁶/° C.,preferably less than 30×10⁻⁶/° C., more preferably less than 25×10⁻⁶/°C., more preferably less than 20×10⁻⁶/° C. Preferably, the film has ashrinkage at 30 mins at 230° C., measured as defined herein, of lessthan 1%, preferably less than 0.75%, preferably less than 0.5%,preferably less than 0.25%, and more preferably less than 0.1%.Preferably, the film has a residual dimensional change ΔL_(r) measuredat 25° C. before and after heating the film from 8° C. to 200° C. andthen cooling to 8° C., of less than 0.75%, preferably less than 0.5%,preferably less than 0.25%, and more preferably less than 0.1%, of theoriginal dimension. In a particularly preferred embodiment, thesubstrate is a heat-stabilised, heat-set biaxially oriented filmcomprising poly(ethylene naphthalate) having the afore-mentionedshrinkage characteristics after 30 min at 230° C., and preferably havingthe afore-mentioned residual dimensional change ΔL_(r) characteristics.

The film may conveniently contain any of the additives conventionallyemployed in the manufacture of polyester films and which are known notto migrate out of the film, to its surface. The additive will nottherefore contaminate the surface of the film during annealing and notcontribute to the observed effect of surface haze. Thus, agents such ascross-linking agents, pigments and voiding agents, agents such asanti-oxidants, radical scavengers, UV absorbers, thermal stabilisers,flame retardants and inhibitors, which are solid, or bound covalently tothe polyester and finally agents which are stable, non-migrating opticalbrighteners, gloss improvers, prodegradents, viscosity modifiers anddispersion stabilisers may be incorporated as appropriate. Inparticular, the film may comprise a particulate filler which can improvehandling and windability during manufacture. The particulate filler may,for example, be a particulate inorganic filler (e.g. voiding ornon-voiding metal or metalloid oxides, such as alumina, silica andtitania, calcined china clay and alkaline metal salts, such as thecarbonates and sulphates of calcium and barium), or an incompatibleresin filler (e.g. polyamides and olefin polymers, particularly a homo-or co-polymer of a mono-alpha-olefin containing up to 6 carbon atoms inits molecule) or a mixture of two or more such fillers.

The components of the composition of a layer may be mixed together in aconventional manner. For example, by mixing with the monomeric reactantsfrom which the film-forming polyester is derived, or the components maybe mixed with the polyester by tumble or dry blending or by compoundingin an extruder, followed by cooling and, usually, comminution intogranules or chips. Masterbatching technology may also be employed.

In a preferred embodiment, the film is optically clear, preferablyhaving a % of scattered visible light (haze) of <10%, preferably <6%,more preferably <3.5% and particularly <1.5%, measured according to thestandard ASTM D 1003. In this embodiment, filler is typically present inonly small amounts, generally not exceeding 0.5% and preferably lessthan 0.2% by weight of a given layer.

One or both surfaces of the polyester film has disposed thereon thecoating materials referred to herein. The coating is preferablyperformed in-line.

In one embodiment, the coating which is to applied to one or bothsurfaces of the polyester film is a hardcoat or scratch resistant layer.The hardcoat layer provides a degree of mechanical protection to thefilm, as judged for example by the Taber abraser test (ASTM MethodD-1044). The Taber Abrasion test will typically cause controlled damageto the surface of unprotected film such that under the standardconditions of treatment, the haze of the film is seen to increase by40-50%. The use of a hardcoat resists the deterioration of the filmsurface under similar conditions and results in an increase in measuredhaze of the material of preferably no more than 20%, more preferably nomore than 10% and most preferably no more than 5%. A further function ofthe hardcoat layer may be to provide a flat, planarised surface to thesubstrate film whose natural surface roughness may vary as a function ofinorganic filler particles present in its composition. Suitable hardcoatlayers which also impart a planarized character to the film surface fallbroadly into one of the three following classifications; organic,organic/inorganic hybrid and predominantly inorganic coats.

Organic hard and planarizing coatings typically comprise (i) aphotoinitiator, (ii) a low molecular weight reactive diluent (e.g amonomeric acrylate), (iii) an unsaturated oligomer (e.g, acrylates,urethane acrylates, polyether acrylates, epoxy acrylates or polyesteracrylates) and (iv) a solvent. As used herein, the term “low molecularweight” describes a polymerisable monomeric species. The term “reactive”signifies the polymerisability of the monomeric species. Such organiccoatings can be cured by free radical reaction, initiated by aphotolytic route. Specific formulations may vary according to thedesired final properties. In one embodiment, the coating compositioncomprises a UV-curable mixture of monomeric and oligomeric acrylates(preferably comprising methylmethacrylate and ethylacrylate) in asolvent (such as methylethylketone), typically wherein the coatingcomposition comprises the acrylates at about 20 to 30 wt % solids of thetotal weight of the composition, and further comprising a minor amount(e.g. about 1% by weight of the solids) of photoinitiator (e.g.Irgacure™ 2959; Ciba).

Organic/inorganic hybrid coatings comprise inorganic particlesdistributed throughout an organic polymeric matrix, which can containcomponent(s) similar to those described immediately above. The coatingsare cured either thermally or by free radical reaction initiated by aphotolytic route, and the presence of a photoinitiator is optional. Theinorganic phase which is often silica or metal oxide particles isdispersed in the polymerisable organic matrix by a number of strategies.In one embodiment, an organic/inorganic hybrid coating comprisesinorganic particles preferably selected from silica and metal oxides;and an organic component comprising a low molecular weight reactivecomponent (e.g. monomeric acrylates) and/or an unsaturated oligomericcomponent (e.g. acrylates, urethane acrylates, polyether acrylates,epoxy acrylates and polyester acrylates); and a solvent, and optionallyfurther comprising a photoinitiator. In a further embodiment, athermally-curable hybrid coating comprises an epoxy resin in combinationwith inorganic (preferably silica) particles which are preferablypresent at a concentration of at least about 10% (preferably at leastabout 20%, and preferably no more than about 75%) by weight of thesolids of the coating composition (which preferably comprises from 5 toabout 20% by weight total solids in alcoholic solution). In a furtherembodiment, a UV-curable hybrid coating composition comprises monomericacrylates (typically multi-functional acrylates) in combination withinorganic (preferably silica) particles in a solvent (such asmethylethylketone), typically wherein the coating composition comprisesthe acrylates and silica at about 5 to 50 wt % solids of the totalweight of the coating composition, and typically further comprising aminor amount (e.g. about 1% by weight of the solids) of photoinitiator.Multi-functional monomeric acrylates are known in the art, and examplesinclude dipentaerythritol tetraacrylate and tris(2-acryloyloxyethyl) isocyanurate.

A predominantly inorganic hardcoat comprises inorganic particles whichare contained in a polymerisable predominantly inorganic matrix such asa polysiloxane. This type of hardcoat is cured thermally.

Suitable examples of a hardcoat and planarizing layer are disclosed in,for instance, U.S. Pat. No. 4,198,465, U.S. Pat. No. 3,708,225, U.S.Pat. No. 4,177,315, U.S. Pat. No. 4,309,319, U.S. Pat. No. 4,436,851,U.S. Pat. No. 4,455,205, U.S. Pat. No. 0,142,362, WO-A-03/087247 and EP1418197 the disclosures of which are incorporated herein by reference.

If present, the coating should preferably have a Tg which is above thetemperature of the subsequent thermal processing or annealing.

In one embodiment, the hardcoat is derived from a coating compositioncomprising:

(a) from about 5 to about 50 weight percent solids, the solidscomprising from about 10 to about 70 weight percent (preferably fromabout 20 to 60 wt %) silica and from about 90 to about 30 weight percentof a partially polymerized organic silanol of the general formulaRSi(OH)₃, wherein R is selected from methyl and up to about 40% of agroup selected from the group consisting of vinyl, phenyl,gamma-glycidoxypropyl, and gamma-methacryloxypropyl, and(b) from about 95 to about 50 weight percent solvent, the solventcomprising from about 10 to about 90 weight percent water and from about90 to about 10 weight percent lower aliphatic alcohol,particularly wherein the coating composition has a pH of from about 3.0to about 8.0, preferably from about 3.0 to about 6.5, preferably lessthan 6.2, preferably about 6.0 or less, and preferably at least 3.5,preferably at least 4.0.

The silica component of the preferred coating composition may beobtained, for example, by the hydrolysis of tetraethyl orthosilicate toform polysilicic acid. The hydrolysis can be carried out usingconventional procedures, for example, by the addition of an aliphaticalcohol and an acid. Alternatively, the silica used in the instantcoating compositions can be colloidal silica. The colloidal silicashould generally have a particle size of about from 5-25 nm, andpreferably about from 7-15 nm. Typical colloidal silicas which can beused in the instant invention include those commercially available as“Ludox SM”, “Ludox HS-30” and “Ludox LS” dispersions (Grace Davison).The organic silanol component has the general formula RSi(OH)₃. At leastabout 60% of the R groups, and preferably about from 80% to 100% ofthese groups, are methyl. Up to about 40% of the R groups can be higheralkyl or aryl selected from vinyl, phenyl, gamma-glycidoxypropyl, andgamma-methacryloxypropyl. The solvent component generally comprises amixture of water and one or more lower aliphatic alcohols. The watergenerally comprises about from 10 to 90 weight percent of the solvent,while the lower aliphatic alcohol complementarily comprises about from90 to 10 weight percent. The aliphatic alcohols generally are thosehaving from 1 to 4 carbon atoms, such as methanol, ethanol, n-propanol,iso-propanol, n-butanol, sec-butanol and tertiary butanol.

In a further embodiment, the coating composition comprises across-linkable organic polymer, for instance a polyethylene imine (PEI),polyester or polyinylalcohol (PVOH), and a cross-linking agent (such asCymel™ 385 or those referred to hereinbelow), in a solvent (typically anaqueous solvent). In this embodiment, the coating composition preferablycomprises PEI (preferably with a molecular weight (Mw) in the range600,000 to 900,000).

The coating compositions can be applied using conventional coatingtechniques, including continuous as well as dip coating procedures. Thecoatings are generally applied at a dry thickness of from about 1 toabout 20 microns, preferably from about 2 to 10 microns, andparticularly from about 3 to about 10 microns. The coating compositioncan be applied either “off-line” as a process step distinct from thefilm manufacture, or “in-line” as a continuation of the filmmanufacturing process. The coating compositions, after application tothe substrate, can be cured at a temperature of from about 20 to about200° C., preferably from about 20 to about 150° C. While ambienttemperatures of 20° C. require cure times of several days, elevatedtemperatures of 150° C. will cure the coatings in several seconds.

The exposed surface of the film may, if desired, be subjected to achemical or physical surface-modifying treatment to improve the bondbetween that surface and a subsequently applied layer. A preferredtreatment, because of its simplicity and effectiveness, is to subjectthe exposed surface of the film to a high voltage electrical stressaccompanied by corona discharge. The preferred treatment by coronadischarge may be effected in air at atmospheric pressure withconventional equipment using a high frequency, high voltage generator,preferably having a power output of from 1 to 20 kW at a potential of 1to 100 kV. Discharge is conventionally accomplished by passing the filmover a dielectric support roller at the discharge station at a linearspeed preferably of 1.0 to 500 m per minute. The discharge electrodesmay be positioned 0.1 to 10.0 mm from the moving film surface.

In a preferred embodiment, the substrate is coated, prior to applicationof the aforementioned coating, with a primer layer to improve adhesionof the substrate to the aforementioned coating composition. The primerlayer may be any suitable adhesion-promoting polymeric composition knownin the art, including polyester and acrylic resins. The primercomposition may also be a mixture of a polyester resin with an acrylicresin. Acrylic resins may optionally comprise oxazoline groups andpolyalkylene oxide chains. The polymer(s) of the primer compositionis/are preferably water-soluble or water-dispersible.

Polyester primer components include those obtained from the followingdicarboxylic acids and diols. Suitable di-acids include terephthalicacid, isophthalic acid, phthalic acid, phthalic anhydride,2,6-naphthalenedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid,adipic acid, sebacie acid, trimellitic acid, pyromellitic acid, a dimeracid, and 5-sodium sulfoisophthalic acid. A copolyester using two ormore dicarboxylic acid components is preferred. The polyester mayoptionally contain a minor amount of an unsaturated di-acid componentsuch as maleic acid or itaconic acid or a small amount of ahydroxycarboxylic acid component such as p-hydroxybenzoic acid. Suitablediols include ethylene glycol, 1,4-butanediol, diethylene glycol,dipropylene glycol, 1,6 hexanediol, 1,4-cyclohexanedimethylol, xyleneglycol, dimethylolpropane, poly(ethylene oxide) glycol, andpoly(tetramethylene oxide) glycol. The glass transition point of thepolyester is preferably 40 to 100° C., further preferably 60 to 80° C.Suitable polyesters include copolyesters of PET or PEN with relativelyminor amounts of one or more other dicarboxylic acid comonomers,particularly aromatic di-acids such as isophthalic acid and sodiumsulphoisophthalic acid, and optionally relatively minor amounts of oneor more glycols other than ethylene glycol, such as diethylene glycol.

In one embodiment, the primer layer comprises an acrylate ormethacrylate polymer resin. The acrylic resin may comprise one or moreother comonomers. Suitable comonomers include alkyl acrylates, alkylmethacrylates (where the alkyl group is preferably methyl, ethyl,n-propyl, isopropyl, n-butyl, isobutyl, t-butyl, 2-ethylhexyl,cyclohexyl or the like); hydroxy-containing monomers such as2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropylacrylate, and 2-hydroxypropyl methacrylate; epoxy group-containingmonomers such as glycidyl acrylate, glycidyl methacrylate, and allylglycidyl ether; carboxyl group or its salt-containing monomers, such asacrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaricacid, crotonic acid, styrenesulfonic acid and their salts (sodium salt,potassium salt, ammonium salt, quaternary amine salt or the like); amidegroup-containing monomers such as acrylamide, methacrylamide, anN-alkylacrylamide, an N-alkylmethacrylamide, an N,N-dialkylacrylamide,an N,N-dialkyl methacrylate (where the alkyl group is preferablyselected from those described above), an N-alkoxyacrylamide, anN-alkoxymethacrylamide, an N,N-dialkoxyacrylamide, anN,N-dialkoxymethacrylamide (the alkoxy group is preferably methoxy,ethoxy, butoxy, isobutoxy or the like), acryloylmorpholine,N-methylolacrylamide, N-methylolmethacrylamide, N-phenylacrylamide, andN-phenylmethacrylamide; acid anhydrides such as maleic anhydride anditaconic anhydride; vinyl isocyanate, allyl isocyanate, styrene,α-methylstyrene, vinyl methyl ether, vinyl ethyl ether, avinyltrialkoxysilane, a monoalkyl maleate, a monoalkyl fumarate, amonoalkyl itaconate, acrylonitrile, methacrylonitrile, vinylidenechloride, ethylene, propylene, vinyl chloride, vinyl acetate, andbutadiene. In a preferred embodiment, the acrylic resin is copolymerisedwith one or more monomer(s) containing oxazoline groups and polyalkyleneoxide chains. The oxazoline group-containing monomer includes2-vinyl-2-oxazoline, 2-vinyl-4-methyl-2-oxazoline,2-vinyl-5-methyl-2-oxazoline, 2-isopropenyl-2-oxazoline,2-isopropenyl-4-methyl-2-oxazoline, and2-isopropenyl-5-methyl-2-oxazoline. One or more comonomers may be used.2-Isopropenyl-2-oxazoline is preferred. The polyalkylene oxidechain-containing monomer includes a monomer obtained by adding apolyalkylene oxide to the ester portion of acrylic acid or methacrylicacid. The polyalkylene oxide chain includes polymethylene oxide,polyethylene oxide, polypropylene oxide, and polybutylene oxide. It ispreferable that the repeating units of the polyalkylene oxide chain are3 to 100.

Where the primer composition comprises a mixture of polyester andacrylic components, particularly an acrylic resin comprising oxazolinegroups and polyalkylene oxide chains, it is preferable that the contentof the polyester is 5 to 95% by weight, preferably 50 to 90% by weight,and the content of the acrylic resin is 5 to 90% by weight, preferably10 to 50% by weight.

Other suitable acrylic resins include:

(i) a copolymer of (a) 35 to 40 mole % alkyl acrylate, (b) 35 to 40%alkyl methacrylate, (c) 10 to 15 mole % of a comonomer containing a freecarboxyl group such as itaconic acid, and (d) 15 to 20 mole % of anaromatic sulphonic acid and/or salt thereof such as p-styrene sulphonicacid, an example of which is a copolymer comprising ethylacrylate/methyl methacrylate/itaconic acid/p-styrene sulphonic acidand/or a salt thereof in a ratio of 37.5/37.5/10/15 mole %, as disclosedin EP-A-0429179 the disclosure of which is incorporated herein byreference; and(ii) an acrylic and/or methacrylic polymeric resin, an example of whichis a polymer comprising about 35 to 60 mole % ethyl acrylate, about 30to 55 mole % methyl methacrylate and about 2 to 20 mole %methacrylamide, as disclosed in EP-A-0408197 the disclosure of which isincorporated herein by reference.

The primer or adherent layer may also comprise a cross-linking agentwhich improves adhesion to the substrate and should also be capable ofinternal cross-linking. Suitable cross-linking agents include optionallyalkoxylated condensation products of melamine with formaldehyde. Theprimer or adherent layer may also comprise a cross-linking catalyst,such as ammonium sulphate, to facilitate the cross-linking of thecross-linking agent. Other suitable cross-linking agents and catalystsare disclosed in EP-A-0429179, the disclosures of which are incorporatedherein by reference.

A further suitable primer, particularly for use in association with thehard-coats described hereinabove, is disclosed in U.S. Pat. No.3,443,950, the disclosure of which is incorporated herein by reference.

The coating of the primer layer onto the substrate may be performedin-line or off-line, but is preferably performed “in-line”, andpreferably between the forward and sideways stretches of a biaxialstretching operation.

The coated and subsequently annealed films preferably have a % ofscattered visible light (haze) of <10%, preferably <6%, more preferably<3.5% and particularly <1.5%, measured according to the standard ASTM D1003 after annealing at a temperature T_(a) (° C.) above Tg whereTg<T_(a)≦Tg+100 (° C.) for a time t after thermal equilibrium where 1hour≦t≦72 hours, and in a specific embodiment after annealing at Tg+80°C. for 30 hours.

In a preferred embodiment, the method of the present invention reduceshaze formation during annealing at a temperature T_(a) (° C.) above Tgwhere Tg<T_(a)≦Tg+100 (° C.) for a time t after thermal equilibriumwhere 1 hour≦t≦72 hours, and particularly during annealing at Tg+80° C.for 30 hours, such that the haze of the annealed coated film ispreferably no more than a 10% increase on the initial average haze valueof the coated film (i.e. before annealing), preferably no more than a 6%increase, preferably no more than a 3.5% increase, preferably no morethan a 1.5% increase, and preferably no more than a 1.0% increase.

In one embodiment of the present invention, the coated and subsequentlyannealed films exhibit a surface having an Ra value, as measured herein,of less than 0.7 nm, preferably less than 0.6 nm, preferably less than0.5 nm, preferably less than 0.4 nm, preferably less than 0.3 nm, andideally less than 0.25 nm, and/or an Rq value, as measured herein, ofless than 0.9 nm, preferably less than 0.8 nm, preferably less than 0.75nm, preferably less than 0.65 nm, preferably less than 0.6 nm,preferably less than 0.50 nm, preferably 0.45 nm or lower, preferablyless than 0.35 nm, and ideally less than 0.3 nm.

It will be appreciated that, in the present invention, the dimensionalstability characteristics described herein refer to the uncoatedheat-stabilised, heat-set, biaxially oriented polyester film.

The coated polyester film is suitable for use as a substrate in anyapplication which requires subsequent processing of the substrate atelevated temperatures, particularly at a temperature T_(a)(° C.) aboveTg where Tg<T_(a)≦Tg+100 (° C.) and particularly for a time t afterthermal equilibrium where 1 hour≦t≦72 hours. Of particular interest isthe use of the coated film as a substrate for, and in the manufactureof, flexible electronic, photonic and optical assemblies or structures,and particularly in the manufacture of the backplanes referred to above.Electronic and opto-electronic devices may comprise a conductivepolymer, and examples include an electroluminescent (EL) device(particularly an organic light emitting display (OLED)), a photovoltaiccell and semiconductor devices (such as organic field effecttransistors, thin film transistors and integrated circuits generally).In one embodiment, the term “electroluminescent display device”, andparticularly the term “organic light emitting display (OLED) device”, asused herein refers to a display device comprising a layer oflight-emitting electroluminescent material (particularly a conductivepolymeric material) disposed between two layers each of which comprisesan electrode, wherein the resultant composite structure is disposedbetween two substrate (or support or cover) layers. In one embodiment,the term “photovoltaic cell” as used herein refers to a devicecomprising a layer of conductive polymeric material disposed between twolayers each of which comprises an electrode, wherein the resultantcomposite structure is disposed between two substrate (or support orcover) layers. In one embodiment, the term “transistor” as used hereinrefers to a device comprising at least one layer of conductive polymer,a gate electrode, a source electrode and a drain electrode, and one ormore substrate layers. Thus, in one embodiment, the method and usereferred to hereinabove include the step of disposing an electrode layeron the coated substrate described hereinabove, in accordance withconventional manufacturing techniques known in the art, and thecomposite film referred to hereinabove further comprises an electrodelayer (optionally transparent or translucent) on the coated substrate.The electrode layer may be a layer, or a patterned layer, of a suitableconductive material as known in the art, for instance gold or aconductive metal oxide such as indium tin oxide. In a furtherembodiment, the composite film described hereinabove may furthercomprise a layer which exhibits barrier properties to water vapourand/or oxygen transmission, particularly such that the water vapourtransmission rate is less than 10⁻⁶ g/m²/day and/or the oxygentransmission rate is less than 10⁻⁵/mL/m²/day, and which is typicallyapplied prior to application of the electrode layer. Such a barrierlayer may be organic or inorganic (preferably inorganic), and istypically applied by vacuum deposition or sputtering techniques.Materials which are suitable for use to form a barrier layer aredisclosed, for instance, in U.S. Pat. No. 6,198,217 and WO-A-03/087247,the disclosures of which are incorporated herein by reference.

Property Measurement

The following approaches were used to characterize the film propertieswhich changed as a consequence of the process described herein:

-   (i) Thermal shrinkage was assessed for film samples of dimensions    200 mm×10 mm which were cut in specific directions relative to the    machine and transverse directions of the film and marked for visual    measurement. The longer dimension of the sample (i.e. the 200 mm    dimension) corresponds to the film direction for which shrinkage is    being tested, i.e. for the assessment of shrinkage in the machine    direction, the 200 mm dimension of the test sample is oriented along    the machine direction of the film. After heating the specimen to the    predetermined temperature (by placing in a heated oven at that    temperature) and holding for an interval of 30 minutes, it was    cooled to room temperature and its dimensions remeasured manually.    The thermal shrinkage was calculated and expressed as a percentage    of the original length.-   (ii) When a sample of film is examined on a flat surface, it can    often exhibit a physical curl. This can arise from its process    history, or through a second, slower creep process under permanent    physical distortion. The curl of a film can be assessed by a simple    physical measurement of the “lift” or height from a flat surface, to    which the edge or corner of a specimen is raised. Thus curl was    measured of samples of film 100 mm×10 mm in dimension, cut in a    specific direction relative to the parent roll (i.e. such that the    100 mm dimension corresponded to the film direction for which the    measurement is desired) and which were laid on a flat, horizontal    surface. The lift was measured for each corner and an average    calculated.-   (iii) For film samples which were essentially transparent, that is    containing sufficiently low levels of additive, pigment, void or    other body which would render it opaque, film clarity was evaluated.    This was achieved by measuring total luminance transmission (TLT)    and haze (% of scattered transmitted visible light) through the    total thickness of the film using a Gardner XL 211 hazemeter in    accordance with ASTM D-1003-61.-   (iv) The glass transition temperature (Tg) of the polyester film was    measured using Differential Scanning Calorimetry (DSC) techniques.    The measurement was performed using a TA Instruments Q100 DSC    System, calibrated using an indium standard. Samples of film were    heated from below ambient temperature (approximately −20° C.) to    300° C. and final values of temperature were reported for a heating    rate of 20° K./minute. The Tg is measured in respect of the    biaxially oriented polyester film before it is exposed to the    annealing process of the invention described herein and, for the    avoidance of doubt, it is this value of Tg which is used to    determine the annealing temperature (T_(a)) of the process.-   (v) Dimensional stability may be assessed in terms of either (a) the    coefficient of linear thermal expansion (CLTE) or (b) a temperature    cycling method wherein the residual change in length along a given    axis is measured after heating the film to a given temperature and    subsequently cooling the film.    -   Both methods of measurements were conducted using a        Thermomechanical Analyser PE-TMA-7 (Perkin Elmer) calibrated and        checked in accordance with known procedures for temperature,        displacement, force, eigendeformation, baseline and furnace        temperature alignment. The films were examined using extension        analysis clamps. The baseline required for the extension clamps        was obtained using a very low coefficient of expansion specimen        (quartz) and the CLTE precision and accuracy (dependent on        post-scan baseline subtraction) was assessed using a standard        material, e.g. pure aluminium foil, for which the CLTE value is        well known. The specimens, selected from known axes of        orientation within the original film samples, were mounted in        the system using a clamp separation of approx. 12 mm and        subjected to an applied force of 75 mN over a 5 mm width. The        applied force was adjusted for changes in film thickness, i.e.        to ensure consistent tension, and the film was not curved along        the axis of analysis. Specimen lengths were normalised to the        length measured at a temperature of 23° C.    -   In the CLTE test method (a), specimens were cooled to 8° C.,        stabilised, then heated at 5° C./min from 8° C. to +240° C. The        CLTE values (α) were derived from the formula:

α=ΔL/(L×T ₂ −T ₁))

-   -   where ΔL is the measured change in length of the specimen over        the temperature range (T₂-T₁), and L is the original specimen        length at 23° C. CLTE values are considered reliable up to the        temperature of the Tg.    -   The data can be plotted as a function of the % change in        specimen length with temperature, normalised to 23° C.    -   In the temperature cycling test method (b), a procedure similar        to that of method (a) was used wherein the temperature was        cycled between 8° C. and several elevated temperatures. Thus,        film samples were heated from 8° C. to 140° C., 160° C., 180° C.        or 200° C. and then cooled to 8° C. The length along each of the        transverse and machine directions was measured at 25° C. before        and after this heat treatment and the change in length ΔL_(r)        calculated as percentage of the original length.

-   (vi) Intrinsic Viscosity (IV)    -   The IV was measured by melt viscometry, using the following        procedure. The rate of flow pre-dried extrudate through a        calibrated die at known temperature and pressure is measured by        a transducer which is linked to a computer. The computer        programme calculates melt viscosity values (log₁₀ viscosity) and        equivalent IVs from a regression equation determined        experimentally. A plot of the IV against time in minutes is made        by the computer and the degradation rate is calculated. An        extrapolation of the graph to zero time gives the initial IV and        equivalent melt viscosity. The die orifice diameter is 0.020        inches, with a melt temperature of 284° C. for IV up to 0.80,        and 295° C. for IV>0.80.

-   (vii) Oxygen transmission rate is measured using ASTM D3985.

-   (viii) Water vapour transmission rate is measured using ASTM F 1249.

-   (ix) Surface Smoothness    -   Surface smoothness is measured using conventional        non-contacting, white-light, phase-shifting interferometry        techniques, which are well-known in the art, using a Wyko NT3300        surface profiler using a light source of wavelength 604 nm. With        reference to the WYKO Surface Profiler Technical Reference        Manual (Veeco Process Metrology, Arizona, US; June 1998; the        disclosure of which is incorporated herein by reference), the        characterising data obtainable using the technique include:    -   Averaging Parameter—Roughness Average (Ra): the arithmetic        average of the absolute values of the measured height deviations        within the evaluation area and measured from the mean surface.    -   Averaging Parameter—Root Mean Square Roughness (Rq): the root        mean square average of the measured height deviations within the        evaluation area and measured from the mean surface.    -   Extreme Value Parameter—Maximum Profile Peak Height (Rp): the        height of the highest peak in the evaluation area, as measured        from the mean surface.    -   Averaged Extreme Value Parameter—Average Maximum Profile Peak        Height (Rpn): the arithmetic average value of the ten highest        peaks in the evaluation area.    -   Extreme Peak Height Distribution: a number distribution of the        values of Rp of height greater than 200 nm.    -   Surface Area Index: a measure of the relative flatness of a        surface.    -   The roughness parameters and peak heights are measured relative        to the average level of the sample surface area, or “mean        surface”, in accordance with conventional techniques. (A        polymeric film surface may not be perfectly flat, and often has        gentle undulations across its surface. The mean surface is a        plane that runs centrally through undulations and surface height        departures, dividing the profile such that there are equal        volumes above and below the mean surface.)    -   The surface profile analysis is conducted by scanning discrete        regions of the film surface within the “field of view” of the        surface profiler instrument, which is the area scanned in a        single measurement. A film sample may be analysed using a        discrete field of view, or by scanning successive fields of view        to form an array. The analyses conducted herein utilised the        full resolution of the Wyko NT3300 surface profiler, in which        each field of view comprises 480×736 pixels.    -   For the measurement of Ra and Rq, the resolution was enhanced        using an objective lens having a 50-times magnification. The        resultant field of view has dimensions of 90 μm×120 μm, with a        pixel size of 0.163 μm.    -   For the measurement of Rp and Rpm, the field of view is        conveniently increased using an objective lens having a 10-times        magnification in combination with a “0.5-times field of view of        multiplier” to give a total magnification of 5-times. The        resultant field of view has dimensions of 0.9 mm×1.2 mm, with a        pixel size of 1.63 μm. Preferably Rp is less than 1000 nm, more        preferably less than 60 nm, more preferably less than 50 nm,        more preferably less than 40 nm, more preferably less than 30        mm, and more preferably less than 20 nm.    -   For the measurement of Ra and Rq herein, the results of five        successive scans over the same portion of the surface area are        combined to give an average value. The data presented below in        respect of Rp are an average value from 100 measurements. The        measurements were conducted using a modulation threshold        (signal:noise ratio) of 10%, i.e. data points below the        threshold are identified as bad data.    -   The surface topography can also be analysed for the presence of        extreme peaks having a height of greater than 200 nm. In this        analysis, a series of measurements of Rp are taken with a pixel        size of 1.63 μm over a total area of 5 cm². The results may be        presented in the form of a histogram in which the data-points        are assigned to pre-determined ranges of peak heights, for        instance wherein the histogram has equally-spaced channels along        the x-axis of channel width 25 nm. The histogram may be        presented in the form of a graph of peak count (y axis) versus        peak height (x axis). The number of surface peaks in the range        300 to 600 nm per 5 cm² area, as determined from Rp values, may        be calculated, and designated as N(300-600). The coatings used        in the present invention preferably result in a reduction of        N(300-600) in the annealed film, such that the reduction F,        which is the ratio of N(300-600) without and with the coating,        is at least 5, preferably at least 15, and more preferably at        least 30. Preferably, the N(300-600) value of the coated and        subsequently annealed film is less than 50, preferably less than        35, preferably less than 20, preferably less than 10, and        preferably less than peaks per 5 cm² area.    -   The Surface Area Index is calculated from the “3-dimensional        surface area” and the “lateral surface area” as follows. The        “3-dimensional (3-D) surface area” of a sample area is the total        exposed 3-D surface area including peaks and valleys. The        “lateral surface area” is the surface area measured in the        lateral direction. To calculate the 3-D surface area, four        pixels with surface height are used to generate a pixel located        in the centre with X, Y and Z dimensions. The four resultant        triangular areas are then used to generate approximate cubic        volume. This four-pixel window moves through the entire        data-set. The lateral surface area is calculated by multiplying        the number of pixels in the field of view by the XY size of each        pixel. The surface area index is calculated by dividing the 3-D        surface area by the lateral area, and is a measure of the        relative flatness of a surface. An index which is very close to        unity describes a very flat surface where the lateral (XY) area        is very near the total 3-D area (XYZ).    -   A Peak-to-Valley value, referred to herein as “PV₉₅”, may be        obtained from the frequency distribution of positive and        negative surface heights as a function of surface height        referenced to the mean surface plane. The value PV₉₅ is the        peak-to-valley height difference which envelops 95% of the        peak-to-valley surface height data in the distribution curve by        omitting the highest and lowest 2.5% of datapoints. The PV₉₅        parameter provides a statistically significant measure of the        overall peak-to-valley spread of surface heights.

The invention is further illustrated by the following examples. Theexamples are not intended to limit the invention as described above.Modification of detail may be made without departing from the scope ofthe invention.

EXAMPLES Comparative Example 1 Preparation of Uncoated Pen Film

A polymer composition comprising PEN was extruded and cast onto a hotrotating polished drum. The film was then fed to a forward draw unitwhere it was stretched over a series of temperature-controlled rollersin the direction of extrusion to approximately 3.1 times its originaldimensions. The draw temperature was approximately 130° C. The film wasthen passed into a stenter oven at a temperature of 135° C. where thefilm was stretched in the sideways direction to approximately 3.4 timesits original dimensions. The biaxially stretched film was then heat-setat temperatures up to 235° C. by conventional means before being cooledand would onto reels. The total thickness was 125 μm. The heat-setbiaxially stretched film was then unwound and then furtherheat-stabilised in a roll-to-roll process by passing the film through anadditional set of ovens, of which the maximum temperature was 190° C.The film was unsupported at its edges and transported through the ovensunder a low line tension, allowing it to relax and stabilize further.

Reference Examples 1-9 Annealing of Uncoated PEN Film

The procedure in comparative example 1 was repeated and a single sheetof the film obtained by that process further treated by annealing in acirculating air oven. The conditions are summarized in Table 1, togetherwith the optical properties (haze and TLT).

TABLE 1 Annealing Annealing Haze TLT Sample Temperature, C. Time, hrs(%) (%) C. Ex. 1 — — 0.82 86.8 Ref. Ex. 1 95 24 1.02 86.9 Ref. Ex. 2 1151 0.97 86.8 Ref. Ex. 3 115 10 1.06 85.2 Ref. Ex. 4 115 24 1.15 86.8 Ref.Ex. 5 115 100 1 86.7 Ref. Ex. 6 135 24 1.77 86.8 Ref. Ex. 7 155 24 5.6386.9 Ref. Ex. 8 175 24 15.06 86.7 Ref. Ex. 9 195 24 30.09 86.2

It is clear that increasing the temperature of annealing results inhigher haze in the film sample. The haze which develops in each sampleis known to be caused by crystals of cyclic oligomer. The oligomerpresent in the bulk of the film diffuses to the surface where itsublimes and crystallizes. This process intensifies at elevatedtemperature and it is clear from the data that haze becomes significantas the temperature increases. For many applications, this surfacedeposit would impair the performance and limit the attractiveness of thefilm. Techniques which are commonly used to clean a film surface wouldotherwise be needed to render useable films which have been annealed athigh temperatures. The present invention addresses the surfacedeposition of oligomeric material during annealing by preventing themigration of the oligomer from the bulk of the film, thereby preventingor minimising surface oligomer deposition.

Control Example 1 Preparation of a Coated Pen Film

The procedure in comparative example 1 was repeated except that the drawratio applied in the direction of extrusion (MD) was increased to 3.3and during the heat setting stage, the transverse dimensions of the webwere reduced by 4%. During manufacture, the film was also treated onboth surfaces with a primer coating, to promote adhesion to asubsequent, thicker coating. The biaxially stretched, heat-set,surface-primed and offline-stabilized film was then unwound and furthermodified on both sides by coating with a material designed to cure to ahard, smooth finish, and again heated, cooled and rewound. The coatingwas of the inorganic hardcoat type described previously and disclosed inWO-A-03/087247. It was prepared before application by the followingsteps:

(i) 517 cm³ of methyltrimethoxysilane (obtained from OSi Specialities)was added to 1034 cm3 demineralised water at room temperature andstirred for 24 hours.(ii) 54 cm³ of 3-glycidoxypropyl trimethoxysilane (obtained from AldrichChemical Company) was added to 108 cm³ of demineralised water at roomtemperature and stirred for 24 hours.(iii) 53 cm³ of 10% aqueous acetic acid (Aldrich Chemical Company) wasadded to 700 cm³ of Ludox LS colloidal silica (12 nm). To this was added162 cm³ of the hydrolysed 3-glycidoxypropyl trimethoxysilane/watermixture and 1551 cm³ of the hydrolysed methyltrimethoxysilane/watermixture. This mixture was stirred for 12 hours before coating. The finalpH of the composition was 6.05.

The coating was applied to both surfaces of the polyester film, to athickness of 3 μm and crosslinked thermally.

Examples 1-9 Annealing the Coated PEN Film of Control Example 1

A single sheet of the film of control example 1 was further treated byannealing in a circulating air oven. The conditions are summarized inTable 2, together with the corresponding optical properties (haze andTLT values).

TABLE 2 Annealing Annealing Sample Temperature, C. Time, hrs Haze (%)TLT (%) Control Ex. 1 — — 0.42 91.4 Example 1 95 24 0.66 91.3 Example 2115 1 0.43 91.4 Example 3 115 10 0.44 91.4 Example 4 115 24 0.47 91.4Example 5 115 100 0.54 91.3 Example 6 135 24 0.45 91.4 Example 7 155 240.47 91.4 Example 8 175 24 0.71 91.4 Example 9 195 24 1.73 90.8

Comparing tables 1 and 2 shows the benefit from an appropriate coatingon each side of PEN film which is subjected to an annealing procedure.The optical properties of the coated film are superior to those of theuncoated film as a result of the coating, and this quality isdemonstrated at annealing temperatures up to 195° C.

To further investigate the use of coatings to reduce the haze producedin the annealing process, the procedure of control example 1 wasrepeated with the coating compositions described below in Examples 10 to16, using PEN and/or PET as the substrate. The PET film was Melinex®ST506 having a thickness of 125 μm, commercially available from DupontTeijin Films. The final dry coating thickness, after curing/drying, was2 μm. The coated film was then annealed in an oven for up to 30 hours atapproximately Tg+80° C. (i.e. 200° C. for a PEN film; 150° C. for a PETfilm) and the haze measured over that period. The following results canbe compared to:

-   -   (i) the uncoated PEN film which exhibited an initial average        haze value (i.e. t=0 h) of 1.4% which increased to 48.8% (an        increase of 47.4%), as shown in the graph of FIG. 1; and    -   (ii) the uncoated PET film which exhibited an initial average        haze value of 0.91% which increased to 41.4% (an increase of        40.5%), as shown in the graph of FIG. 2,        when annealed under these conditions. Average haze values were        calculated by taking the average of three values measured across        the width of the film.

Example 10

An organic coating composition comprising a mixture of monomeric andpolymeric acrylates (including methylmethacrylate and ethylacrylate) anda photoinitiator (Irgacure™ 2959; Ciba) in a solvent of methyl ethylketone (2-butanone) was prepared at 26.5 wt % solids (of which about 1%of these solids is the photoinitiator) to a viscosity of about 1.22 cP(centipoise). The coating was dried at 80° C. and then cured byUV-radiation. The haze measurements of the coated film after annealingfor up to 30 hours are shown in the graphs of FIGS. 3 and 4. The initialaverage haze values of the PEN and PET films were 0.74% and 0.48%,respectively.

Example 11

A hybrid organic/inorganic coating composition comprising acrylatemonomers and silica particles in MEK solvent was prepared to 10% solidsand a viscosity of about 1.7 cP. The coating was applied and then curedimmediately by UV-radiation. The haze measurements of the coated filmafter annealing for up to 30 hours are shown in the graphs of FIGS. 5and 6. The initial average haze values of the PEN and PET films were0.88% and 0.53%, respectively.

Example 12

The coating composition of control example 1 was coated onto the PETsubstrate referred to above, and the haze measurements of the coatedfilm after annealing for up to 30 hours are shown in the graph of FIG.7. The initial average haze of the film was 0.50%.

Example 13

A coating comprising polyethylene imine (Sigma Aldrich code 181978-8;average molecular weight Mw of about 750,000) and a crosslinker (Cymel™385) in water at approximately 5% by weight PEI solids, was coated ontothe substrate and thermally cured at 180° C. The haze measurements ofthe coated film after annealing for up to 30 hours are shown in Tables 3and 4 below.

TABLE 3 PEI on PEN Time Average Increase (%) (h) Haze (%) in haze 0 1.270 1 1.27 0 6 1.30 0.03 11 1.36 0.09 17 1.41 0.14 23 1.43 0.16 30 1.420.15

TABLE 4 PEI on PET Time Average Increase (%) h Haze (%) in haze 0 4.58 01 5.43 0.85 6 5.58 1.00 11 6.38 1.80 17 6.28 1.70 23 6.81 2.23 30 7.803.22

Example 14

A thermally-curable coating composition comprising epoxy resin incombination with silica particles present at a concentration of about41% by weight of the solids of the coating composition, which in turncomprises about 10% by weight total solids in an alcoholic solution (amixed solvent system of isopropanol, n-butanol, ethanol andcyclohexanone). The composition is stirred for 6 hours at roomtemperature, coated and then thermally cured at 180° C. The hazemeasurements of the coated films after annealing for up to 30 hours areshown in the graphs of FIGS. 8 and 9. The initial average haze values ofthe PEN and PET films were 0.65% and 0.45%, respectively.

Example 15

A thermally-curable coating comprising polyester (TPE 62C; Takemoto Oiland Fat Company, Japan), a crosslinker (Cymel™ 385; Cytec) in aqueoussolvent (8% total solids, of which 86% is the polyester) was coated ontothe PEN substrate and thermally cured at 180° C. The haze measurementsof the coated film after annealing for up to 30 hours are shown in Table5 below.

TABLE 5 Time Average Increase (%) (h) Haze (%) in haze 0 1.06 0 1 0.83−0.23 6 3.87 2.82 11 5.76 4.70 17 7.35 6.29 23 8.16 7.11 30 9.46 8.40

Example 16

A coating composition comprising PVOH (Airvol™ 24-203; Air Products) at24% by weight of the coating composition, a surfactant (Caflon™ NP10;Uniqema) at 10% by weight of the coating composition and varying amounts(0 g, 17, 24 and 29% by weight of the PVOH present in the composition)of crosslinking agent (Cymel™ 350; American Cyanamid), in aqueoussolvent, was coated onto the PEN substrate and thermally cured at 180°C. The haze measurements of the coated films after annealing for up to30 hours are shown the graph of FIG. 10. The initial average haze valuesof the coated PEN films were 0.73% (no crosslinker), 0.74% (9%crosslinker), 0.76% (17% crosslinker), 0.59% (24% crosslinker) and 0.8%(29% crosslinker).

Examples 10 to 16 provide further demonstration of the effectiveness ofthe coatings described herein to reduce the formation of haze during anabove-Tg annealing process, relative to the uncoated films.

1. A method of preventing or minimising the formation of haze in abiaxially oriented polyester film during annealing of said film aboveits glass transition temperature (Tg (° C.)), comprising disposing onone or both surfaces of said film a coating derived from a compositionselected from the group consisting of: (i) an organic coatingcomposition comprising a low molecular weight reactive diluent; anunsaturated oligomer; a solvent; and a photoinitiator; (ii) anorganic/inorganic hybrid coating composition comprising a low molecularweight reactive component and/or an unsaturated oligomeric component; asolvent; and inorganic particles, and optionally further comprising aphotoinitiator; (iii) a predominantly inorganic hardcoat coatingcomposition comprising inorganic particles contained in a polymerisablepredominantly inorganic matrix; and (iv) a coating compositioncomprising a cross-linkable organic polymer selected from polyethyleneimine (PEI), polyester and polyvinylalcohol (PVOH), and a cross-linkingagent; and subsequently annealing said film above the glass transitiontemperature (Tg (° C.)).
 2. (canceled)
 3. The method according to claim1, wherein said annealing is conducted at a temperature T_(a) (° C.)above Tg where Tg<T_(a)≦Tg+100 (° C.).
 4. The method according to claim1, wherein said annealing is conducted for a time t after thermalequilibrium where 1 hour≦t≦72 hours, and then film then cooled.
 5. Themethod according to claim 4, wherein 1 hour≦t≦48 hours.
 6. The methodaccording to claim 4, wherein 1 hour≦t≦24 hours.
 7. The method accordingto claim 1, wherein a coating is derived from a composition selectedfrom the group consisting of: (i) an organic coating compositioncomprising a low molecular weight reactive diluent selected frommonomeric acrylates; an unsaturated oligomer selected from acrylates,urethane acrylates, polyether acrylates, epoxy acrylates and polyesteracrylates; a solvent; and a photoinitiator; (ii) an organic/inorganichybrid coating composition comprising a low molecular weight reactivecomponent selected from monomeric acrylates and/or an unsaturatedoligomeric component selected from acrylates, urethane acrylates,polyether acrylates, epoxy acrylates and polyester acrylates; a solvent;and inorganic particles selected from silica and metal oxides, andoptionally further comprising a photoinitiator; and (iii) apredominantly inorganic hardcoat coating composition comprisinginorganic particles contained in a polymerisable predominantly inorganicmatrix selected from a polysiloxane.
 8. The method use according toclaim 1, wherein a coating is derived from a composition selected fromthe group consisting of: (i) an organic coating composition comprising alow molecular weight reactive diluent selected from monomeric acrylates;an unsaturated oligomer selected from urethane acrylates, polyetheracrylates, epoxy acrylates and polyester acrylates; a solvent; and aphotoinitiator; (ii) an organic/inorganic hybrid coating compositioncomprising a low molecular weight reactive component selected frommonomeric acrylates and/or an unsaturated oligomeric component selectedfrom urethane acrylates, polyether acrylates, epoxy acrylates andpolyester acrylates; a solvent; and inorganic particles selected fromsilica and metal oxides, and optionally further comprising aphotoinitiator; and (iii) a predominantly inorganic hardcoat coatingcomposition comprising inorganic particles contained in a polymerisablepredominantly inorganic matrix selected from a polysiloxane.
 9. Themethod according to claim 1, wherein a coating is derived from aUV-curable composition comprising monomeric and oligomeric acrylates,and a photoinitiator.
 10. The method according to claim 1, wherein acoating is derived from a UV-curable composition comprising monomericacrylates, silica particles and a photoinitiator.
 11. The methodaccording to claim 1, wherein a coating is derived from a compositioncomprising: (a) from about 5 to about 50 weight percent solids, thesolids comprising from about 10 to about 70 weight percent silica andfrom about 90 to about 30 weight percent of a partially polymerizedorganic silanol of the general formula RSi(OH)₃, wherein R is selectedfrom methyl and up to about 40% of a group selected from the groupconsisting of vinyl, phenyl, gamma-glycidoxypropyl, andgamma-methacryloxypropyl, and (b) from about 95 to about 50 weightpercent solvent, the solvent comprising from about 10 to about 90 weightpercent water and from about 90 to about 10 weight percent loweraliphatic alcohol, wherein the coating composition has a pH of fromabout 3.0 to about 8.0.
 12. The method according to claim 1, wherein acoating is derived from a thermally-curable composition comprising anepoxy resin and silica particles.
 13. The method according to claim 1,wherein a coating is derived from a composition comprising across-linkable organic polymer selected from a polyethylene imine (PEI),polyester and polyvinylalcohol (PVOH), and further comprising across-linking agent.
 14. The method according to claim 1, wherein acoating has a dry thickness of from 1 to 20 microns.
 15. The methodaccording to claim 1, wherein the annealed and coated film exhibits ahaze value of less than 10%.
 16. The method according to claim 1,wherein, after annealing at a temperature T_(a) (° C.) above Tg whereTg<T_(a)≦Tg+100 (° C.) for a time t after thermal equilibrium where 1hour≦t≦72 hours, the coated and annealed film exhibits a haze valuewhich is no more than a 10% increase on the haze value of the coatedfilm before annealing.
 17. A biaxially oriented composite filmcomprising a polyester substrate supporting one or more coatings on oneor both surfaces of said substrate, wherein said one or more coatingsis/are derived from a coating composition selected from the groupconsisting of: (i) an organic coating composition comprising a lowmolecular weight reactive diluent; an unsaturated oligomer; a solvent;and a photoinitiator; (ii) an organic/inorganic hybrid coatingcomposition comprising a low molecular weight reactive component and/oran unsaturated oligomeric component; a solvent; and inorganic particles,and optionally further comprising a photoinitiator; and (iii) a coatingcomposition comprising a cross-linkable organic polymer selected frompolyethylene imine (PEI), polyester and polyvinylalcohol (PVOH), and across-linking agent; wherein said polyester substrate is aheat-stabilised, heat-set, biaxially oriented film which exhibits one ormore of: (i) a shrinkage at 30 mins at 230° C. of less than 1%; (ii) aresidual dimensional change ΔL_(r) measured at 25° C. before and afterheating the film from 8° C. to 200° C. and then cooling to 8° C., ofless than 0.75%; and/or (iii) a coefficient of linear thermal expansion(CLTE) within the temperature range from −40° C. to +100° C. of lessthan 40×10⁻⁶/° C.
 18. The film according to claim 17, wherein at leastone coating is derived from a composition selected from the groupconsisting of: (i) an organic coating composition comprising a lowmolecular weight reactive diluent selected from monomeric acrylates; anunsaturated oligomer selected from acrylates, urethane acrylates,polyether acrylates, epoxy acrylates and polyester acrylates; a solvent;and a photoinitiator; and (ii) an organic/inorganic hybrid coatingcomposition comprising a low molecular weight reactive componentselected from monomeric acrylates and/or an unsaturated oligomericcomponent selected from acrylates, urethane acrylates, polyetheracrylates, epoxy acrylates and polyester acrylates; a solvent; andinorganic particles selected from silica and metal oxides, andoptionally further comprising a photoinitiator.
 19. The film accordingto claim 17, wherein at least one coating is derived from a compositionselected from the group consisting of: (i) an organic coatingcomposition comprising a low molecular weight reactive diluent selectedfrom monomeric acrylates; an unsaturated oligomer selected from urethaneacrylates, polyether acrylates, epoxy acrylates and polyester acrylates;a solvent; and a photoinitiator; and (ii) an organic/inorganic hybridcoating composition comprising a low molecular weight reactive componentselected from monomeric acrylates and/or an unsaturated oligomericcomponent selected from urethane acrylates, polyether acrylates, epoxyacrylates and polyester acrylates; a solvent; and inorganic particlesselected from silica and metal oxides, and optionally further comprisinga photoinitiator.
 20. The film according to claim 17, wherein at leastone coating is derived from a UV-curable composition comprisingmonomeric and oligomeric acrylates, and a photoinitiator.
 21. The filmaccording to claim 17, wherein at least one coating is derived from aUV-curable composition comprising monomeric acrylates, silica particlesand a photoinitiator.
 22. The film according to claim 17, wherein atleast one coating is derived from a thermally-curable compositioncomprising an epoxy resin and silica particles.
 23. The film accordingto claim 17, wherein at least one coating is derived from a compositioncomprising a cross-linkable organic polymer selected from a polyethyleneimine (PEI), polyester and polyvinylalcohol (PVOH), and furthercomprising a cross-linking agent.
 24. The film according to claim 17,wherein said at least one coating has a dry 10 thickness of from 1 to 20microns.
 25. The method according to claim 1, wherein said polyester ispoly(ethylene naphthalate) or poly(ethylene terephthalate).
 26. Themethod according to claim 1, wherein said polyester is poly(ethylenenaphthalate).
 27. The method according to claim 26, wherein saidpolyester is derived from 2,6-naphthalenedicarboxylic acid.
 28. Themethod, use according to claim 1, wherein the poly(ethylene naphthalate)has an intrinsic viscosity of 0.5-1.5.
 29. The method according to claim1, wherein said biaxially oriented polyester film is a heat-stabilised,heat-set, biaxially oriented film.
 30. The method according to claim 29,wherein said polyester is poly(ethylene terephthalate).
 31. The methodaccording to claim 29, wherein said heat-stabilised, heat-set, biaxiallyoriented film exhibits one or more of: (i) a shrinkage at 30 mins at230° C. of less than 1%; (ii) a residual dimensional change ΔL_(r)measured at 25° C. before and after heating the film from 8° C. to 200°C. and then cooling to 8° C., of less than 0.75%; and/or (iii) acoefficient of linear thermal expansion (CLTE) within the temperaturerange from −40° C. to +100° C. of less than 40×10⁻⁶/° C.
 32. Thecomposite film according to claim 17, wherein the film exhibits a hazevalue of less than 10% after annealing at a temperature T_(a)(° C.)above Tg where Tg<T_(a)≦Tg+100 (° C.) for a time t after thermalequilibrium where 1 hour≦t≦72 hours.
 33. The composite film according toclaim 17, wherein, after annealing at a temperature T_(a) (° C.) aboveTg where Tg<T_(a)≦Tg+100 (° C.) for a time t after thermal equilibriumwhere 1 hour≦t≦72 hours, the coated film exhibits a haze value which isno more than a 10% increase on the initial haze value of the coated filmbefore annealing.
 34. The film according to claim 17, wherein saidpolyester is poly(ethylene terephthalate).
 35. The film according toclaim 17, wherein said polyester is poly(ethylene naphthalate).
 36. Thefilm according to claim 35, wherein said polyester is derived from2,6-naphthalenedicarboxylic acid.
 37. The film according to claim 35,wherein the poly(ethylene naphthalate) has an intrinsic viscosity of0.5-1.5.
 38. The method according to claim 29, wherein said polyester ispoly(ethylene naphthalate).
 39. The method according to claim 38,wherein said polyester is derived from 2,6-naphthalenedicarboxylic acid.40. The method according to claim 38, wherein the poly(ethylenenaphthalate) has an intrinsic viscosity of 0.5-1.5.